1. Field of the Invention
This invention generally relates to a linear motion guide unit assembly, and, in particular, to a linear motion rolling contact guide unit assembly having a mechanism for absorbing a misalignment, such as a deviation from a parallel arrangement of two opposed surfaces.
2. Description of the Prior Art
An automatic centering flat surface sliding bearing is well known in the art as described in the Japanese Patent Post-exam Pub. No. 53-21058, and it includes a bearing main body having an outer peripheral surface defined by a straight cylindrical surface and a sliding member mounted on the outer peripheral surface of the main body slidably in the circumferential direction. However, this prior art technique can be applied only to a specific flat surface sliding bearing having a specific structure and is not always applicable to a common linear motion guide unit.
On the other hand, among casted articles, in particular casted articles from aluminum, there are those which gradually deform and develop internal distortions with time after casting, though they may be minor. For example, as shown in FIG. 13, in building a relatively large sliding table assembly including a base 40, a table 41 and a pair of linear motion rolling contact guide units 42 and 43 spaced apart from each other over a lateral distance of d, the base 40 is formed with a pair of coplanar lower mounting surfaces 44 and 45 and the table 41 is also formed with a pair of coplanar upper mounting surfaces 46 and 47, and these coplanar lower and upper mounting surfaces 44-45 and 46-47 are set in parallel to each other. A linear motion rolling contact guide unit is well known in the art and it generally includes an elongated rail, a slider slidably mounted on the rail and a plurality of rolling members, such as balls or rollers, interposed between the rail and the slider. Typically, the rail is rectangular in cross section and provided with a pair of inner guide grooves each on each side of the rail. The slider is generally U-shaped in cross section and mounted on the rail in a straddling fashion and the slider is provided with a pair of outer guide grooves, each located opposite to a corresponding one of the pair of inner guide grooves to thereby define a load path section. The guide unit may be either of the infinite motion type or the finite motion type and in the case of the infinite motion type, the slider is provided with a pair of endless circulating paths in which the rolling members are provided to roll therealong endlessly.
However, if the base 40 and the table 41 are left untouched after casting them and machining them to form the lower and upper mounting surfaces 44-45 and 46-47, for example, only the base 40 may be deformed as shown in FIG. 14 to provide a generally convex top surface so that the lower mounting surfaces 44 and 45 come to be inclined in the opposite directions with respect to the horizonal plane. As a result, the lower mounting surfaces 44 and 45 are not coplanar any more and they are not in parallel with the upper mounting surfaces 46 and 47, even if they may remain coplanar from each other.
Under the condition, if a rail 48 of the linear motion guide unit 42 is fixedly attached to the lower mounting surface 44 and similarly a rail 49 of the linear motion guide unit 43 is fixedly attached to the other lower mounting surface 45, a wedge-shaped gap 52 will be created between a slider 50 of the linear motion guide unit 42 and the upper mounting surface 46 and similarly another wedge-shaped gap 53 will be created between a slider 51 of the other linear motion guide unit 43 and the other upper mounting surface 53.
The base 40 may be deformed to present a concave top surface in which case similar wedge-shaped gaps will be created but in the opposite directions than shown in FIG. 14. Besides, there may be a case in which only the table 41 is deformed, or both of the base 40 and the table 41 are deformed. In each of these cases, a wedge-shaped gap of the kind described above and shown in FIG. 14 will be created, though a particular shape and its size may differ depending on the various conditions. In order to cope with this situation, it is conceivable to prepare a wedge-shaped shim plate to plug such a wedge-shaped gap; however, since the particular shape and size differs one from another, it is not possible to prepare appropriate shim plates in advance. Thus, it is not a viable technique to cope with the situation. Another possible approach would be to machine the mounting surfaces again, but such a reprocessing would be extremely time-consuming and it could be a source for pushing up the cost.